This invention relates to a thin film waveguide path which is formed of organic high molecular material with extremely low beam propagation loss.
A low loss wavegide path is known in the art which comprises a thin film waveguide path of acryl series resin coated on a quartz glass substrate that is optically polished, by a spin coat method (e.g. Optical and Quantum Electronics, Vol. 7, 1975, pages 443-446). A further known known art is waveguide path comprises a polymethyl methacrylate (PMMA) substrate and a nitrocellulose thin film waveguide path layer (e.g. Applied Physics Letters, Vol. 29, No. 5; Sept. 1, 1976, pages 303-304).
The first-mentioned waveguide path is not only convenient as it is formed with the spin coat method, but also has the advantage of very reduced value of 0.13 dB/cm of the beam propagation loss of 0.13dB/cm. However, the cost of second-mentioned materials to implement the waveguide path is very expensive.
On the other hand, the second-mentioned waveguide path has low materials cost but has the drawback of high beam propagation loss. An additional known waveguide path is disclosed Japanese Laid-Open Patent Publication No. 59/1984-151107 in which the substrate of the waveguide path is formed of the high molecular compound, and a beam waveguide path is formed of an organic compound or high molecular compound on the substrate. However, such beam waveguide path suffers from the drawback of high beam propagation loss.
As discovered by the present inventors, the beam propagation loss of a waveguide path formed by applying a spin coat on a plastic substrate cannot be lowered, regardless of how carefully the the spin coat, typically comprising an organic high molecular material is prepared.
High molecular materials for beam waveguide paths, such as polycarbonate (PC), polystyrene (PSt), PMMA and polysulfone (PSF), which are transparent plastics, are generally close together in solubility parameters and dissolve well in hydrocarbon halides, such as dichloromethane and chloroform, and also in hydrocarbons such as tetrahydrofuran (THF) and benzene. When such a high molecular material is dissolved in the foregoing solvents and coated on an optically polished glass substrate, the coated thin film exhibits good uniformity and it is easy to produce a thin film beam waveguide path with a propagation loss of less than 1 dB/cm. However, when the transparent plastic is coated on a plastic substrate (e.g., PC, PMMA) by employing hydrocarbon halide as a solvent, it is difficult if not impossible to produce no matter how carefully the process is controlled, an excellent light waveguide path of low light propagation loss less than 20 dB/cm; and, hence, such transparent material is unfit for use as a light waveguide path.